1. Field of the Invention
This invention relates to apparatus for protecting an electrical load such as a motor and the wiring supplying power to the motor from overload currents. More particularly, it relates to the combination of a contactor for switching the motor off and on with a circuit breaker that not only has a trip function for protecting the wiring, but also opens the contactor as an overload trip is approached to thereby provide motor protection previously provided by a separate overload relay.
2. Background Information
It is common to control power to electrical loads such as motors with a contactor. In its simplest form, the contactor can be just an “off/on” switch such as a manually operated mechanical switch. More commonly, the contactor is an electrically operated switch in which a solenoid is continuously energized to hold the contacts closed. The contactor is opened by spring force when the coil is de-energized. In many applications, overcurrent protection is provided for the load. This is particularly true for loads such as motors that draw current well above rated current (maximum continuous current) during starting. An overload relay models the heat generated by this overload current as an I2t function. As long as the current stays within the bounds of this function, with an appropriate margin, the contactor remains closed. Should the current limits be exceeded because of a stalled rotor or other reason, the overload relay trips the contactor open by de-energizing the solenoid. Thus, the overload relay protects the load.
The electrical system providing power to the load also includes a circuit breaker upstream of, and often at a considerable distance from, the contactor. The circuit breaker protects the wiring, typically from short circuits and from excessive current drawn by the load. Short circuit protection is provided by an instantaneous and/or short delay trip function that opens the circuit breaker without appreciable delay if the current exceeds a selected very high amplitude. A long delay trip function may be included to respond to persistent overload conditions typically through use of an I2t function to protect the wiring.
Currently three devices are used to control and protect loads such as motors: a contactor, an overload relay, and a circuit breaker. The contactor is the primary device for energizing and de-energizing the load, and as such, is designed for many thousands of operations. The circuit breaker provides protection, and will experience fewer operations. The circuit breaker is often located remotely from the load.